Propylene glycol monomethyl ether propionate compound and the process for the preparation thereof

ABSTRACT

A propylene glycol monomethyl ether propionate compound is obtained by the esterification of propylene glycol monomethyl ether with propionic acid at an elevated temperature above 80 °C. in the presence of acidic catalyst and azeotropic agent. This compound is rectified for removing acid residue and moisture in order to yield the desirable product having high purity.

FIELD OF THE INVENTION

This invention relates to propylene glycol monomethyl ether propionatehaving the formulas ##STR1## obtained by the reaction between propyleneglycol monomethyl ether and propionic acid.

BACKGROUND OF THE INVENTION

Organic ester compounds are good solvent widely used in synthetic resinindustry, such as for paints, inks, adhesives and detergents. Atpresent, ether compounds are mainly divided into two categories, i.e. Eseries and P series. The E series of ether compounds are obtained fromthe synthesis of alcohols and ethylene oxide while P series of ethercompounds are obtained from the synthesis of alcohols and propyleneoxide. The related ester compounds thereof are primarily acetate esters,whereas no propionate ester products have been developed so far. Inrecent years, it has been found that the E series of ethers or acetateester compounds thereof when inhaled into human body are likelydecomposed into alkoxy acetic acid ether and in turn induce theerythrocyte becoming abnormal and thus toxic to the genital organ. Onthe other hand, P series would not cause these damages. At present,propylene glycol monomethyl ether acetate of P series is inferior to thesolubility against unsaturated polyesters or polyurethane resins as wellas the drying capability of the coating thereof. Therefore, it isurgently demanded to develop a kind of solvent having not only excellentsolubility and drying capability but also low toxicity.

Directing to this requirement, the inventors of the present applicationendeavored in positive research and development, and eventually producedsuccessfully propylene glycol monoethyl ether propionate with bestresult.

SUMMARY OF THE INVENTION

An object of the present application is to provide a novel propyleneglycol monomethyl ether propionate compound.

Another object is to provide a process for the preparation of propyleneglycol monomethyl ether propionate (called "PMP" hereunder), whichcomprises reacting propylene glycol monomethyl ether (called "PMG"hereunder) obtained by synthesis reaction of methanol and propyleneoxide under high pressure at high temperature, and propionic acid in thepresence of acidic catalyst.

Further object is to the PMP as a solvent, especially in the fields ofpaints, inks, adhesives and detergents industries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a NMR'H spectrum of PMP produced according to the presentinvention; and

FIG. 2 shows a mass spectrum of PMP produced according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process for the preparation of PMP according to the presentinvention consists of batch and continuous processes. In general whenthe moisture generated during the reaction is not removed, the reactionsystem is likely reached an equilibrium state which would hinder theincrease of productivity and thus disadvantageous to industrialproduction. Therefore, in the process of the present invention, nomatter whether batch or continuous process is adopted, the reactionsystem may be added with aromatic compound as an azeotropic agent. Theazeotropic component so added must be incompatible with water and hasazeotropic effect with water. Aromatic compound is good in meeting theserequirement.

In the batch process, the reactants and azeotropic solvent are placedinto the reactor for carrying out the reaction at the azeotropictemperature while removing the water and recycling the azeotropicsolvent. After the end of the reaction, the product and azeotropicsolvent are separated by fractionating distillation so as to obtain theproduct of high purity.

In the continuous process, the starting materials are fed at a givenflow rate on the end hand, and the water is withdrawn continuousely fromthe top of the fractionator during process of the reaction on the otherhand. The azeotropic solvent is recycled while the reaction systemcontaining the product PMP in a concentration of a certain extentpresented in the reactor is transferred into a rectifying tower in orderto proceed the fractionating distillation to remove a minor amount ofunreacted ether, acid and PMP. Thereby, a product of high purity isobtained.

According to the process for the preparation of PMP in the presentapplication, the starting materials of PGM and propionic acid arereacted at an elevated temperature above 80° C. in the presence ofacidic catalyst and azeotropic agent for taking place the esterificationto produce crude PMP. The product is then separated by rectification forremoving the unreacted acid and generated water to obtain PMP of highpurity.

For the starting material, the molar ratio of PGM to propionic acid isgenerally in the range of 0.6 to 3.0, preferably from 1.1 to 1.5, inwhich PGM is in excess with respect to propionic acid. If the molarratio is smaller than 0.6 or greater than 3.0, after the reaction isaccomplished, either party in the reaction system will leave unreactedresidue in great excess so that not only more energy consumption will berendered during the rectifying process, but also the rectifying timerequired is increased so as to decrease the production. If propionicacid is excess too much, namely the molar ratio smaller than 0.6, thereaction rate will be remarkably dropped. The reason is unclear but itis assumed that the acidic catalyst is subjected to buffering.

The catalysts used in the present invention include inorganic acids,such as sulfuric acid, hydrochloric acid and phosphoric acid, andorganic acids, such as acetic acid, oxalic acid, citric acid, p-toluenesulfonic acid and methane sulfonic acid, among which the strong acidsincluding sulfuric acid, p-toluene sulfonic acid or methane sulfonicacid is preferred. Propylene glycol monomethyl ether propionate obtainedaccording to the present application has a boiling point of 160.5° C.,while other reactants and products having respective boiling point of120° C. for PGM, 140.8° C. for propionic acid and 100° C. for water. Itis apparently easier to rectify this solution comparing the respectiveboiling point of 146° C. for propylene glycol monomethyl ether acetate,118° C. for acetic acid and 120° C. for PGM as in the conventional case.This is one of the effects achieved by the process according to thepresent invention.

Another effect achieved is lower toxicity of PMP against the metabolicorgans. In accordance with NOEL (NO Observable Effect Level) publishedby Environment Protection Agency of U.S.A., E and P series of esters forrabbit are 30 and 3000 ppm, respectively. PMP of this invention belongsto P series and thus has very low toxicity against to genital organ.

Further, PMP has better solubility to various resins. For example,propylene glycol monomethyl ether acetate has final solvent percentagesof 70% to alkyd resin while present PMP is over 90%, an excellentsolvent is herewith proved.

In the process of the present invention, when the reaction is completedthe reaction system must be treated by double rectification. Primaryrectification is a dehydration and deacidification procedures. Forenhancing the efficiency and saving the energy, an azeotropic agentselected from aromatic organic solvent comprising benzene, toluene,xylene and cyclohexane is added in amount of 6 to 30% with respect tocombined starting materials in order to reduce the azeotropictemperature in rectification. At secondary rectification, only twocomponents of tether and ester are left, which are likely completelyfractionally distilled based on the difference of boiling points betweentwo components, thereby a product of high purity is obtained.

Now, the present invention will be further described by means of thefollowing Examples which are merely for the purpose of illustration andby no means of any limitation therefor.

EXAMPLE 1

Into the reator having a volume of 3 liters, 1172 ml of PGM and 746 mlof propionic acid were introduced. After mixing 200 ml of xylene and 10grams of p-toluene sulfonic acid were added. Then the temperature wasbrought to reflux temperature at 142° C. to carry out the reaction for 5hours. During this period the dehydration was taken place simultaneouslyin favor of the progression of the reaction. The reaction solution wasanalyzed by gas chromatography and found the following composition:

    ______________________________________                                        PMP             69.91%                                                        PGM             15.34%                                                        propionic acid   6.33%                                                        xylene           7.78%                                                        water            0.59%                                                        ______________________________________                                    

This solution was further treated by double rectification, PMP compoundhaving a purity greater than 99.9% was obtained.

This PMP compound was characterized by NMR'H spectrum as shown in FIG. 1and mass spectrogram as shown in FIG. 2, thereby the chemical structurethereof can be determined as ##STR2##

EXAMPLE 2

Into the reactor having a volume of 3 liters, 1172 ml of PGM and 746 mlof propionic acid were introduced. After mixing 10 grams of P-toluenesulfonic acid were added. Then the temperature was brought to 99.1° C.the azeotropic temperature of water and propionic acid, to carry out thereflux reaction for 5 hours. Since no azeotropic solvent was added, theaqueous layer was impossibly separated, therefore the equilibrium waslikely reached. This reaction solution was analyzed by gaschromatography and found the following composition:

    ______________________________________                                        PMP             44.6%                                                         PGM             27.6%                                                         propionic acid  22.4%                                                         water            5.4%                                                         ______________________________________                                    

COMPARATIVE EXAMPLE

Into the reactor having a volume of 3 liters, 1172 ml of PGM and 572 mlof acetic acid were introduced. After mixing, 10 grams of P-toluenesulfonic acid were added. Then the temperature was brought to the refluxtemperature of 97.5° C. to carry out the reaction for 5 hours. Thereaction solution was analyzed by gas chromatography and found thefollowing composition:

    ______________________________________                                        propylene glycol monomethyl ether acetate                                                              44.0%                                                acetic acid              20.1%                                                PGM                      29.7%                                                water                     5.8%                                                ______________________________________                                    

This solution was treated by double rectification, propylene glycolmonomethyl ether acetate having a purity of 99% was obtained. The yieldis too low to meet the inductrial requirement.

What I claim is:
 1. Propylene glycol monomethyl ether propionatecompound obtained by the reaction between propylene glycol monomethylether and propionic acid.
 2. A process for the preparation of propyleneglycol monomethyl ether propionate, comprising the steps of reactingpropylene glycol monomethyl ether and propionic acid at elevatedtemperature above 80° C. in the presence of acidic catalyst andazeotropic agent for taking place the esterification to produce crudepropylene glycol monomethyl ether propionate and removing the unreactedacid and water by rectification to obtain desirable propylene glycolmonomethyl ether propionate having high purity.
 3. The process as setforth in claim 2, wherein the molar ratio between propylene glycolmonomethyl ether and propionic acid is in the range from 0.6 to 3.0. 4.The process as set forth in claim 2, wherein said catalyst is selectedfrom a group consisting of sulfuric acid, P-toluene sulfonic acid andmethane sulfonic acid.
 5. The process as set forth in claim 2, whereinsaid azeotropic agent is selected from a group consisting of benzene,toluene, xylene and cyclohexane, in an amount of 6% to 30% by volumewith respect to the sum of propylene glycol monomethyl ether andpropionic acid.
 6. Use of propylene glycol monomethyl ether propionateas the solvent for paints, inks, adhesives and detergents.